2009-12-28_PERMIT FILE - C1996083

• Permit No: C1996083
• IBM Index Class Name: Permit File
• Doc Date: 12/28/2009
• Doc Name: 5/18/2009 Revised Stability Evaluation of Coal Mine Refuse Disposal Area No. 4, BuckhornGeotech,
• Section_Exhibit Name: Volume IX Coal Mine Waste Disposal Area No. 2
• Media Type: D
• Archive: No

• extremely difficult. Similar colluvial material in the area has performed well at steep slopes.
Additionally, excavations in similar colluvial soils at Refuse Pile No. 2 have stood at slopes of
1H:1V or steeper for several months. Sedimentary bedrock outcrops can be observed on the
slope above the proposed refuse pile. These nearly level units provide stability to the slope
above the proposed refuse pile. The unit weight of the foundation soils was estimated to be
125 pcf.
Table 3. Material Properties Used in the Stability Evaluations
Material
Type
Unit
Weight
Effective Stress
Parameters
deg.)
C.
Ps
Coal Mine Refuse
110
34.0
0
Native Foundation Soil
125
34.0
100
Notes:
1. Sensitivity analyses conducted for �'= 30.0 and 32.0 degrees
5.3 Pore Pressure Conditions
Pore pressures within the coal waste bank were conservatively modeled by incorporating a
phreatic surface into the limit equilibrium stability model. Pore pressures in compacted coal
refuse at the adjacent CWDA No. 2 are being monitored by vibrating wire piezometers. These
installations and monitoring records are documented by BGI (2006 and 2008). The vibrating
wire piezometers at CWDA No. 2 demonstrate relatively low pore pressures with pore pressure
dissipation over time, and therefore do not represent the undrained conditions that a total
stress analysis evaluates. Furthermore, the pore pressures measured by the piezometers are
well below the pore pressures at failure that were measured during the CU triaxial shear tests
at similar confining stresses as the piezometers. For these reasons, effective stress analyses
using approximated pore pressures measured at CWDA No. 2 were used for these stability
evaluations.
Although pore pressures are not currently being monitored at CWDA No.4, a phreatic surface
was approximated for the stability evaluations based on monitoring at CWDA No. 2. To account
for uncertainty, a sensitivity analysis was conducted with respect to the location of the assumed
phreatic surface. The sensitivity analysis provides an evaluation of the potential development of
a higher phreatic surface than described above. Figure 2 illustrates the phreatic surfaces used
for the updated stability analyses at CWDA No. 4. The assumed high phreatic surface is
considered very conservative and is not anticipated to be realized.
r 1
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Bowie No 2 Refuse Pile No 4 2009 Revised stability.doc
Project #06 -481 -003
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